The present invention concerns an apparatus for the arrangement of membranes in a module and, in particular, a plate module for the separation of mixtures by a pervaporation process.
Separation processes using membranes are, today, implemented on a large scale in many technical fields, such as sea and brackish water desalination, treatment of process solutions and for ultrafiltration. In other fields, however, such as for gas separation and the separation of organic liquid mixtures, the implementation of membranes is in its early stages. The latter process, the separation of organic liquid mixtures by means of membranes, is known as pervaporation and it differs from other membrane processes in particular through the formation of vaporous permeate.
Pervaporation is, more specifically, a separation process in which a liquid feed mixture to be separated is brought into contact with a first or feed side of a suitable membrane. If, at the second or permeate side of the membrane, the partial vapor pressures of the components of the liquid feed mixture are kept below the partial vapor pressures of the components at the feed side, a driving force for the migration of these components through the membrane is established. According to their respective permeabilities in the membrane, the components of the feed mixture pass through the membrane at different rates; in fact, permeation rates are complex functions of a number of different parameters, including the nature and concentration of the permeating species, the constituency and structure of the membrane, the composition of the mixture, temperature, and others. Since the partial vapor pressures are lower at the permeate side than at the feed side of the membrane, the components evaporate after passing through the membrane, forming a vaporous permeate (hence the term "pervaporation"). Due to the differing permeabilities of the feed-mixture components, the composition of the permeate differs from that of the feed mixture, and a separation of the feed mixture is observed.
If a sufficiently high difference in partial vapor pressures is maintained between the feed side and the permeate side, the separation capacity of a given membrane vis-a-vis a given feed mixture is determined only by the permeabilities of the respective components. Only nonporous membranes can be used for pervaporation, and it is believed that the solubility of a component in the membrane material, together with diffusivities, governs mass transport across the membrane. Thus, pervaporation must not be mistaken for membrane distillation. The latter is a normal distillation process, using porous membranes, and separation follows thermodynamic vapor-liquid equilibria. By contrast, pervaporation is a nonequilibrium dynamic process, where transport phenomena determine separation efficiency.
With pervaporation, as with the other membrane separation processes, the mixture to be separated is brought into contact with one side of a suitable membrane. But the differences in the chemical potentials of the mixture components on both sides of the membrane which are necessary for permeation, and thus separation, are not created in the case of pervaporation, by an increase in pressure (and consequent increasing of the chemical potential) on the mixture side as with other membrane processes, but by a lowering of the chemical potentials on the permeate side. This is most simply achieved through the creation of a vacuum on the permeate side, such that the partial pressures of all components are decreased on the permeate side of the membrane to levels below the corresponding partial pressures on the mixture side. Constant pumping off of the components passing through the membrane in a vaporous form or their condensation in a vacuum, enables constant maintaining of a sufficiently large difference in the partial pressure on both sides of the membrane. Obviously, only those components of a mixture which possess a sufficiently high volatility can permeate through a pervaporation membrane.
For pervaporation processes, as indicated above, poreless membranes are used. The varying permeation rates of the components of the mixture to be separated is the sole factor which determines the separation effectiveness of the pervaporation process. No thermodynamic equilibrium is established between the two sides of the membrane. The selectivity and the extent of the achievable separation are solely determined by the transport characteristics of the membrane. Accordingly, mixtures such as azeotropes can be effectively separated, which is not possible utilizing thermodynamic vapor-liquid equilibria.
The literature provides the expert in the field with descriptions of modules that have been proposed for implementing pervaporation processes (e.g., Stuckey, U.S. Pat. No. 2,958,656; Stuckey, U.S. Pat. No. 2,958,657; Kirkland, U.S. Pat. No. 3,182,043; Thijsen et al, U.S. Pat. No. 3,367,787; Martin et al, U.S. Pat. No. 3,140,256). Modules in accordance with these proposals, as well as attempts to use modules like those successfully employed in reverse osmosis and ultrafiltration processes, have not been successful, however, and have failed to lead to industrial implementation of pervaporation processes. None of these modules meets the specific requirements and particularities of the pervaporation process sufficiently to enable economic implementation thereof.
The arrangement of hollow fiber membranes in a bundle is not successful in the case of pervaporation, since the loss in pressure of the vapor on the permeate side in the narrow inner holes of a hollow fiber rapidly exceeds a tolerable level. Pressure loss calculations show that, with the implementation of normal hollow fibers for pervaporation, a fiber length of 20 cm should not be much exceeded, in order to enable successful execution of the pervaporation process, provided that all other conditions are also favorable.
For the same reasons, the normal spiral-wound module, where the permeate, after passing through the membrane, flows spirally through a narrow, pressure-proofed channel to a permeate collection pipe, cannot be used for pervaporation.
European Patent Application No. 0 096 340, however, describes a module with a spirally-wound membrane, in which, contrary to the conventional spiral wound modules, the flow direction and channels for the raw feed and the permeate are reversed, so that the permeate, after passing through the membrane, has to travel, at most, a distance of half a module length in an open channel before leaving the module. Such a module can be used successfully for pervaporation.
Due to their simple construction and the possibility of also implementing small-surface membranes in them, plate modules play a considerable role in membrane technology. Thus, West German Offenlegungsschrift No. 3 304 956 describes a plate module for pervaporation which is constructed to achieve an alignment of heat chambers, raw feed chambers and permeate chambers. The heating of each raw feed chamber is devised to maintain a constant operating temperature throughout the module and to replace immediately heat losses caused by the pervaporation process. The trans-membrane flow of a pervaporation membrane doubles on the one hand, with a temperature increase of approximately 10.degree. K, while the vaporizing permeate, on the other hand, withdraws heat from the system and, thereby, from the raw feed.
However, the additional expenditure in material for the heating chamber, the reduced packing density and the necessity of additional sealing off of the raw feed chambers against the heat chambers, along with the resulting additional sources for defects and leaks, are not offset, relative to the available membrane surface, by the slightly increased average flow.
Furthermore, according to West German Offenlegungsschrift No. 3 304 956 the permeate vapor is first directed from the individual permeate chambers through special channels within the closed module. This means that considerable pressure losses on the permeate side have to be tolerated, which greatly limits the implementation possibilities of such a module, or that only small module units with smaller membrane surface and enlarged permeate channels can be used, whereby the global packing density is again reduced and the module becomes much more costly.